The Serious Fraud Office has closed an investigation into one of New Zealand's largest privately owned construction companies, Hawkins Construction Limited, in relation to a major million Auckland power upgrade project.

The SFO said despite "unusual" business conduct, there were no grounds to pursue a criminal prosecution.

In August, the SFO served search warrants on Hawkins Construction offices, including the Hobson Street substation site at the centre of a $419 million network upgrade on behalf of state-owned Transpower and listed infrastructure company Vector.

SFO acting chief executive Simon McArley said the investigation had not yielded evidence of criminal activity, but there were oddities in the case.

"While the manner in which the parties conducted their business appears to us to be unusual, we have evidence from industry experts that it is entirely possible that those involved honestly believed the conduct was legitimate.

"In light of that it cannot be concluded that the criminal threshold has been reached," McArley said.

McArley said the complaint was made in good faith and it was important for businesses to be truthful and honest in their dealings.

"Attempts to disguise, bury or fudge the true nature of transactions can easily risk crossing the line into a criminal deception," he said.

Spokespeople for Vector and Hawkins were not immediately able to comment.